Hundreds of thousands of native fish, including Murray cod, have died this weekend in the third mass death event in a matter of weeks.
A sharp temperature drop from over 40 degrees to under 30 degrees overnight has caused layers of water with different oxygen levels to mix, reducing the oxygen available and suffocating the fish.
Department of primary industries have sent fisheries officers to assess the situation in the Menindee weir pool and neighbouring sections of the Darling river.